Why was the book ' WEB ', by John Wyndham, published ten years after his death?

John Wyndham, author of Day of the Triffids amongst others, died in 1969.

Ten years after his death his estate allowed the novella Web to be published for the first time. (along with a couple of other books not released during his lifetime)

Why wasn't the book published during his lifetime? (or if this was the last title he wrote, why the 10 year delay?)

The book doesn't seem to be incomplete and, although a little short, after reading I didn't see a reason why it wouldn't have been released (bearing in mind the great success that several other of his novels had - I'm sure a publisher would have known that any release from him would have sparked interest and profit).

The story, to a reader in 2014, is a little clichéd but I don't know if that would have been the case when written (and I don't know when he actually wrote the story). Besides, a few clichés doesn't get in the way of a riveting read.

Thanks